Calculations and the Chemical Equation Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 5 Calculations and the Chemical Equation Denniston Topping Caret 4th Edition

5.1 The Mole Concept and Atoms The Mole and Avogadro’s Number atomic mass unit (amu) - unit of measure for the mass of atoms. carbon-12 assigned the mass of exactly 12 amu 1 amu = 1.66 x 10-24 g periodic table gives atomic weights in amu. What is the atomic weight of one atom of Fluorine? 1

5.1 The Mole Concept and Atoms Answer: 19.00 amu What would be the mass of this one atom in grams? 5.1 The Mole Concept and Atoms Chemists usually work with much larger quantities. It is more convenient to work with grams than amu.

5.1 The Mole Concept and Atoms To make the connection we must define the mole. The mole is abbreviated mol 1 mol of atoms = 6.022 x 1023 atoms of an element 5.1 The Mole Concept and Atoms Avegadro’s number A mole is simply a unit that defines an amount of something Just as a dozen defines 12 Just as a gross defines 144

5.1 The Mole Concept and Atoms The average mass of one atom of an element in amu is numerically equivalent to the mass of one mole of an element expressed in grams. That is, 1 atom F is 19.00 amu and 1 mole of F is 19.00 g. Or, 19.00 amu/atom F 19.00 g/mole F 2 5.1 The Mole Concept and Atoms =19.00 g F/mol F or 19.00 g/mol F

5.1 The Mole Concept and Atoms Molar mass - The mass in grams of 1 mole of atoms. What is the molar mass of carbon? 12.01 g/mol C This means if you counted out a mole of Carbon atoms (i.e, 6.022 x 1023 of them) they would have a mass of 12.01 g. 5.1 The Mole Concept and Atoms

5.1 The Mole Concept and Atoms Calculating Atoms, Moles, and Mass 2 We now know the following conversion factors: Conversion Tool g  ml density amu  g 1 amu = 1.661 x 10-24 g 6.022 x 1023 amu = 1g moleactual number Avegadro’s No. g  mole Molar Mass 5.1 The Mole Concept and Atoms

5.1 The Mole Concept and Atoms Let's Practice Using these Tools 1. What is the mass in grams of 2.5 mol Na. 2. Calculate the number of atoms in 1.7 mol B. 3. Calculate the number of atoms in 5.0 g Al. 4. Calculate the mass of 5,000,000 atoms of Au. 5.1 The Mole Concept and Atoms

Figure 5.2

5.2 Compounds 3 The Chemical Formula Chemical Formula - a combination of symbols of the various elements that make up the compound. Formula unit - the smallest collection of atoms that provide two important pieces of information the identity of the atoms and the relative number of each type of atom

5.2 Compounds Let’s look closely at the following formulas: H2O NaCl Fe(CN)3 (NH4)3PO4 CuSO4.5H2O This is an example of a hydrate - compounds containing one or more water molecules as an integral part of their structure. 5.2 Compounds

5.3 The Mole Concept Applied to Compounds 4 Formula weight - the sum of the atomic weights of all atoms in the compound, as represented by its formula. expressed in amu Molar mass applies to compounds also. What is the formula weight of H2O? 16.00 amu + 2(1.008 amu) = 18.02 amu What is the molar mass of H2O? 18.02 g/mol H2O

5.3 The Mole Concept Applied to Compounds When calculating the formula weight (or molar mass of an ionic compound, use the smallest unit of the crystal 5.3 The Mole Concept Applied to Compounds What is the molar mass of (NH4)3PO4? 149.10 g/mol (NH4)3PO4

5.4 The Chemical Equation and the Information It Conveys A Recipe For Chemical Change Chemical Equation - shorthand notation of a chemical reaction. Reactants - (starting materials) - the substances that undergo change in the reaction. Products - substances produced by the reaction.

Law of Conservation of Mass - matter cannot be either gained or lost in the process of a chemical reaction. The total mass of products must equal the total mass of the reactants. 5.4 The Chemical Equation

5.4 The Chemical Equation Features of a Chemical Equation  - means heat is needed. Products and reactants must be specified using chemical symbols 5.4 The Chemical Equation Reactants- written on the left of arrow Products - written on the right Physical states are shown in parentheses

Coefficient - how many of the substance are in the reaction The equation must be balanced. All the atoms of every reactant must also appear in the products. How many Hg’s on left? on right? How many O’s on left? 5.4 The Chemical Equation

The Experimental Basis of a Chemical Equation We know that a chemical equation represents a chemical change. The following is evidence for a reaction: Release of a gas. CO2 is released when acid is placed in a solution containing CO32- ions. H2 is released when Na is placed in water. Formation of a solid (precipitate.) A solution containing Ag+ ions is mixed with a solution containing Cl- ions. 5.4 The Chemical Equation

Heat is produced or absorbed (temperature changes) Acid and base are mixed together The color changes Light is absorbed or emitted Changes in the way the substances behave in an electrical or magnetic field Changes in electrical properties. 5.4 The Chemical Equation

5.5 Balancing Chemical Equations 6 Consider the following reaction: hydrogen reacts with oxygen to produce water Write the above reaction as a chemical equation. You probably wrote the following: H2 + O2  H2O Don’t forget the diatomic elements.

5.5 Balancing Chemical Equations H2 + O2  H2O Is the law of conservation of mass obeyed as written? NO Balancing chemical equations uses coefficients to ensure that the law of conservation of mass is obeyed. You may not change subscripts! WRONG: H2 + O2  H2O2 5.5 Balancing Chemical Equations

5.5 Balancing Chemical Equations H2 + O2  H2O Using the above equation, let’s see the steps to balancing: Step 1. Count the number of moles of atoms of each element on both product and reactant side. 5.5 Balancing Chemical Equations Reactants Products 2 mol H 2 mol H 2 mol O 1 mol O

5.5 Balancing Chemical Equations H2 + O2  H2O Step 2. Determine which elements are not balanced. Oxygen is not balanced. Step 3. Balance one element at a time using coefficients. 5.5 Balancing Chemical Equations H2 + O2  2H2O This balances oxygen, but is Hydrogen still balanced?

5.5 Balancing Chemical Equations H2 + O2  2H2O How will we balance hydrogen? 2H2 + O2  2H2O 5.5 Balancing Chemical Equations Step 4. Check! Make sure the Law of conservation of mass is obeyed. Reactants Products 4 mol H 4 mol H 2 mol O 2 mol O

5.5 Balancing Chemical Equations Let's Practice Balancing Balance the following equations: 1. C2H2 + O2  CO2 + H2O 2. AgNO3 + FeCl3  Fe(NO3)3 + AgCl 3. C2H6 + O2  CO2 + H2O 4. N2 + H2  NH3 5.5 Balancing Chemical Equations

5.6 Calculations Using the Chemical Equation 7 We will learn in this section to calculate quantities of reactants and products in a chemical reaction. Need a balanced chemical equation for the reaction of interest. Keep in mind that the coefficients represent the number of moles of each substance in the equation.

5.6 Calculations Using the Chemical Equation Let’s examine the reaction: 2H2 + O2  2H2O What do the coefficients tell us? 2 mol H2 reacts with 1 mol O2 to produce 2 mol H2O. What if 4 moles of H2 reacts with 2 moles of O2? It yields 4 moles of H2O 5.6 Calculations Using the Chemical Equation

5.6 Calculations Using the Chemical Equation 2H2 + O2  2H2O The coefficients of the balanced equation are used to convert between moles of substances. Let’s see how: How many moles of O2 are needed to react with 4.26 moles of H2? You may be able to do this in your head, but let’s see how to use the factor label method. 5.6 Calculations Using the Chemical Equation

5.6 Calculations Using the Chemical Equation 2H2 + O2  2H2O 1 2.13 mol O2 5.6 Calculations Using the Chemical Equation 2 Comes from the balanced equation.

5.6 Calculations Using the Chemical Equation Conversion Tool g  ml density amu  g 1 amu = 1.661 x 10-24 g moleactual number Avegadro’s No. g  mole Molar Mass mol A mol B Coef. of balanced eq. 5.6 Calculations Using the Chemical Equation

5.6 Calculations Using the Chemical Equation Plan your route before calculating.

5.6 Calculations Using the Chemical Equation Let's do some examples. Na + Cl2  NaCl 1. Balance the equation. 2. Calculate the moles of Cl2 which will react with 5.00 mol Na. 3. Calculate the number of grams of NaCl which will be produced when 5.00 mol Na reacts with an excess of Cl2. 4. Calculate the grams of Na which will react with 5.00 g Cl2. 5.6 Calculations Using the Chemical Equation

5.6 Calculations Using the Chemical Equation Theoretical and Percent Yield 7 Theoretical yield - the maximum amount of product that can be produced Pencil and paper yield Actual yield - the amount produced when the reaction is performed Laboratory yield Percent yield: 5.6 Calculations Using the Chemical Equation

5.6 Calculations Using the Chemical Equation Another example Given the reaction: CaH2(s) + 2H2O(l)  Ca(OH)2(aq) + 2H2(g) Calculate the theoretical yield (in grams) of H2 given that 5.00 g CaH2 reacted with 6.00 g H2O. Calculate the % yield if 0.20 grams was collected in the laboratory when the above amounts were reacted. 5.6 Calculations Using the Chemical Equation

The End Chapter 5